This invention relates generally to circuits and methods for operating bilateral direct current control systems, and more particularly, to the operation of four-arm bridge circuits which control the amplitude and direction of current through loads, in response to pulse-width modulated signals.
One known system for controlling load current by the use of a four-arm bridge, each such arm having a switching transistor electrically disposed therein, is disclosed in "Siemens-Zeitschrift" 43 (1969), No. 5, pages 458 to 464. In this prior art arrangement, each of the arms of the bridge contains an electronic switching element. In operation, a first pair of electronic switching elements which are on diagonally disposed arms of the bridge circuit with respect to one another, are periodically and simultaneously closed and opened, while the remaining second pair of diagonally disposed switching elements remains open. Reversal of the current through the load is achieved by simultaneously opening and closing the second pair of switching elements, while the first pair remains opened. This method of operating a four bridge arm control circuit has the advantage of a linear relationship between the control voltage and the load voltage. Moreover, such a drive system may be simply implemented.
It is a disadvantage of the foregoing system that the polarity of the load voltage and the current which flows into the bridge changes during the opening intervals of the periodically operated electronic switches. This results because the inductive load component causes load current to remain flowing during the intervals that the operated switches are opened. Such current flows through bypass diodes which are disposed in shunt across each such electronic switch. Such diode current flows back into the power supply, in a direction opposite to the current flow during the time that such switches are closed. Load voltage is reversed concurrently with such bypass diode current. The effect of this operation is that the load will experience a current having a relatively large alternating current ripple component which produces additional heat loss in the load. Moreover, in situations where a motor is used as the load, the large alternating current component can create whining noises.
One prior art method for reducing the amplitude of the alternating current component in the load current by the use of pulse-width control is disclosed in U.S. Pat. No. 3,260,912. In this known system, a first pair of diagonally disposed electronic switching elements in the bridge are opened and closed during time intervals which are offset with respect to one another. Thus, during the open interval of each such electronic switching element, the diagonally associated electronic switching element remains closed. This offset driving arrangement provides an advantage over the hereinabove discussed simultaneous driving arrangement because the load current which continues to flow after a particular switching element is opened, as a result of the inductive component of the load, does not flow back into the power supply, but circulates through the closed electronic switching element and a bypass diode. This arrangement, therefore, produces after each cycle of switching elements closure which delivers to the load electrical energy from the power supply, a bypass phase which is distinguishable from the energy reversing backflow phase of the previously discussed arrangement, which does not reverse the load voltage, but reduces it to zero. Thus, the alternating current ripple component in the motor load current has the same pulse frequency as in the first-mentioned arrangement, but only one-half of the magnitude. This arrangement, however, has the disadvantage of a non-linear relationship between the control voltage and the load voltage, particularly in the range of small control voltages. Thus, it is possible in situations where small control voltages are utilized, that the load voltage would approach zero before the control voltage approaches zero. This results from the fact that the conductive intervals of diagonally disposed electronic switching elements cannot overlap because a safety interval must be maintained to prevent electronic switching elements which are disposed on the same half of the bridge from being simultaneously conductive and causing short circuit conditions. Similarly, an overlap of the conductive intervals of diagonally opposite switching elements can occur when negative control voltages are utilized only when such control voltages exceed predetermined negative values.
It becomes apparent, therefore, that a bilateral direct current control bridge, which is operated in accordance with the latter method described hereinabove, has a region of insensitivity in which the load voltage is zero for small control voltages. This operational characteristic is a significant disadvantage for most applications. The electronic switching elements of the second diagonal, which are conductive during the non-conductive interval of the said first pair of electronic switching elements, do not carry any current. The current is conducted through the shunt bypass diodes which are poled for conduction in a direction which is opposite to the forward conduction of the respectively associated switching elements. This may cause, in some applications, damage to the switching elements.
Accordingly, it is an object of this invention to provide an arrangement for controlling a bilateral control element in such a manner that the alternating current ripple component is minimized.
It is a further object of this invention to provide an arrangement for controlling a control bridge in a manner which achieves linearity between the control and load voltages.